Before the mending, where Planeswalkers the only characters to travel between planes without another party's help? Were there beings that could travel the planes but didn't have the Planeswalker Spark?
Nope! We had the Skyship Weatherlight, which was interplanar, and carried several non-Planeswalkers; we had Thran based teleporters; Soul Conduits (I believe that's what they're called) to capture beings and transport them to another world; and there was the Rathi Overlay which was sort of a form of Planeswalking. Sort of. If you squint.
Basically any being of sufficient power could move between planes (Elder Dragons, Myojin of Night's Reach, Demonic Leviathan, Void Maws), and then there were technological methods for doing so developed by the Phyrexians and later Urza.
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